Dear Pruya Dips

I suggest to work with temperature dependent heat capacity. like this :

  1. Cp = A+BT+CT2

   2. Tave = (T1 + T2 )/2

for more information about " Variation of Natural Gas Heat Capacity with Temperature, Pressure, and Relative Density " and find above constants you could pay attention to below link.

Best Regards

http://www.jmcampbell.com/tip-of-the-month/2009/07/variation-of-natural-gas-heat-capacity-with-temperature-pressure-and-relative-density/

Thank you very much for the reply Amir. I did come across this correlation. Are you able to provide some insights on the applicability of this correlation for multiphase where certain gas components can exist as liquid and gaseous phase?

Couple of comments:

At higher pressures (say, above 200 psi), heat capacity becomes a function of temperature and pressure. I don't know what you're trying to do, but I would define the problem in terms of enthalpy differences, and then use an equation of state (Redlich-Kwong for example). That way, you also avoid problems with liquids, since enthalpy allows a  direct formulation with the 1st law of thermodynamics (for all phases).